Reverse issues on 904 transmission

After sitting for 24 years, you may need more than band adjustments. Try the band adjustments first, but after all that time sitting, the lip seals in the drums could need replacing. When you have in your case, a gear that doesn't work very well until warm, that is usually a sign that the lip seals are hard, and only soften when the oil warms up. As much as I hate to say it, a transmission overhaul could be in the near future.
Save time and money -> DO IT RIGHT - ONCE.